1. Field of the Invention
This invention relates to a circuit interrupter for a power circuit and, more particularly, to a motor starter which provides low cost current limiting protection for a motor.
2. Background of Information
Circuit interrupters are electrically operated switches used for controlling motors and other types of electrical loads. Circuit interrupters include, for example, contactors, motor starters, motor controllers and other electromechanical switching devices. Electromagnetic contactors, for example, include a plurality of movable electrical contacts which are brought into contact with a plurality of fixed electrical contacts to close the contactor and connect a power line to the load. The movable contacts are separated from the fixed contacts to open the contactor.
Contactors also include a magnetic circuit having an electromagnet and a movable armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage for electromagnetically accelerating the pole of the armature towards the pole of the electromagnet, thus reducing the air gap. Disposed on the armature are the movable contacts. The complementary fixed contacts are fixedly disposed within the contactor case and engage the movable contacts as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are interconnected with the fixed contacts and become interconnected with each other as the movable contacts make with the fixed contacts.
A contactor with an overload protection relay system is called a motor starter. The purpose of the overload relay is to estimate the heat produced in the motor by line current and "trip" or stop the motor if the retained heat exceeds an acceptable level. State of the art overload protection relay systems include current sensors which output a voltage proportional to the line current. The current sensor output voltage, in turn, charges a capacitor over one line cycle. As disclosed in U.S. Pat. Nos. 5,270,898 and 5,315,471, the voltage across the capacitor is converted to a digital value by a successive approximation analog-to-digital (A/D) converter. After the A/D conversion, a microprocessor squares and integrates the digital value to achieve a true measure of motor heating.
U.S. Pat. No. 4,893,102 discloses an electromagnetic contactor which controls energization of a contactor coil in four separate stages: (1) an acceleration stage; (2) a coast stage; (3) a grab stage; and (4) a hold stage. When at rest, the contacts are held in a normally open position by the force of a kickout spring disposed within the contactor assembly. In the acceleration stage, the contactor coil is fully energized and the contacts are accelerated toward a closed position at a maximum rate. In the coast stage, the contactor mechanism has already achieved enough velocity to achieve closure, so energization of the contactor coil is reduced or eliminated entirely to reduce the force of contact closure impact to a minimum level. In the grab stage, the system evaluates the closing velocity of the contactor mechanism and adjusts energization of the contactor coil to ensure the contactor mechanism has enough momentum to guarantee contact closure. Finally, in the hold stage, energization of the contactor coil is reduced to a level sufficient to counteract the force of the kickout spring and maintain the contacts in a closed position.
U.S. Pat. No. 5,128,825 is directed to an electromagnetic contactor which accommodates to dynamic conditions of the contactor coil and supply voltage. The contactor gates a first voltage pulse to the coil of the contactor electromagnet at a fixed, preferably full, conduction angle, and monitors the electrical response of the coil, namely the peak current. The conduction angle of the second pulse is then adjusted based upon the peak current produced by the first voltage pulse and the voltage of the first pulse to provide, together with the first voltage pulse, a constant amount of electrical energy to the coil despite variations in coil resistance and supply voltage. The third and subsequent voltage pulses to the coil of the contactor are gated at conduction angles preselected in order that, with constant energy supplied by the first and second voltage pulses, the contacts touch and then seal at a substantially constant point in a selected pulse. Contact closure can occur at the third pulse, or in a large contactor where more energy is required, at a later pulse. Contact touch and sealing consistently occur on declining coil current in order to achieve low impact velocity and reduced contact bounce.
Normally, the third and subsequent pulses are gated to the contactor coil at constant, preselected conduction angles. However, under marginal conditions for closure where the peak current produced by the first voltage pulse is below a predetermined value, a second set of conduction angles is used to gate the third and subsequent voltage pulses to the coil. This second set of conduction angles produces a substantially full conduction of the third and subsequent pulses.
It is known to use a current regulator system in an electromagnetic contactor for regulating coil current to a current reference. It is also known to drive the coil with a field effect transistor (FET) drive circuit. The FET drive circuit is switched by a pulse width modulated (PWM) signal. The PWM signal normally switches current to the coil for a predetermined percentage of the period of the PWM signal. The coil current is sensed and compared to the current reference. Whenever the sensed coil current exceeds the current reference, the PWM signal is disabled. In turn, the FET drive circuit is disabled for the remainder of the period of the PWM signal and, thus, the coil current is regulated to the current reference.
There is a need for an improved circuit interrupter having a reduced cost with respect to prior art circuit interrupters.
There is a more particular need for such a circuit interrupter which simplifies the analog-to-digital conversion circuitry for a plurality of load currents.
There is another more particular need for such a circuit interrupter which simplifies the drive circuitry for the electromagnetic coil.